The present invention provides a process for quenching a gaseous reaction mixture during the phosgenation of diamines in the gas phase to produce diisocyanates, wherein the gas mixture contains at least diisocyanate, phosgene and hydrogen chloride. Quenching is achieved by injecting a quenching liquid into the gas mixture.
The preparation of diisocyanates by reacting diamines with phosgene in the gas phase is described, for example, in EP 0 289 840. The diisocyanates formed in a cylindrical reaction chamber, such as a tubular reactor, are not thermally stable at the reaction temperatures of 300 to 500xc2x0 C. Rapid cooling of the reaction gases after the phosgenation reaction to temperatures below 150xc2x0 C. is therefore needed to avoid the formation of undesired secondary products due to the thermal decomposition of diisocyanate or by further reaction. For this purpose, in EP 0 289 840, the gaseous mixture continually leaving the reaction chamber, which contains, inter alia, diisocyanate, phosgene and hydrogen chloride, is passed into an inert solvent, e.g. dichlorobenzene. The disadvantage of this process is that the rate of flow at which the gas mixture is passed through the solvent bath has to be relatively low because at too high rates of flow the solvent and the compounds dissolved therein would be carried over. In a subsequent step, the liquid compounds have to be separated from the gas. Another disadvantage is that, due to the low rates of flow and a small heat transfer term, large solvent containers have to be used to produce the cooling effect.
Furthermore, processes are known which use heat exchangers and/or expand the gases into a vacuum to cool the reaction gases. The disadvantage of heat exchangers is that, due to poor heat transfer, large exchange surfaces and thus large heat exchangers are required for effective cooling. In addition, deposits of solids on the relatively cold surfaces of the heat exchangers takes place due to secondary reactions of the gas mixture on these surfaces, such as e.g. decomposition or polymerization. The transfer of heat is further impaired by these deposits and this leads to a higher residence time and thus results in a further increase in secondary product formation. On top of that, undesired shutdown times are produced for the entire plant due to cleaning of the cooling stage.
The present invention reduces or eliminates the disadvantages inherent in the art such as those mentioned above when rapidly cooling the gaseous reaction mixture present during the gas phase phosgenation of diamines to produce diisocyanates to a temperature at which the relevant reaction product is thermally stable. At the same time, the formation of undesired secondary products is suppressed.